extmk.rb: include UPDATE_LIBRARIES
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48204 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
extmk.rb: include UPDATE_LIBRARIES
which does not include macro definitions given by command line
in MAKEFLAGS.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48204 b2dd03c8-39d4-4d8f-98ff-823fe69b080e